Polyimides constitute a class of valuable polymers being characterized by thermal stability, inert character, usual insolubility in even strong solvents, and high Tg, among others. Their precursors are usually polyamic acids, which may take the final imidized form either by thermal or by chemical treatment.
Polyimides have always found a large number of applications requiring at least some of the aforementioned characteristics in numerous industries, and recently their applications have started increasing dramatically in electronic devices, especially as dielectrics. With continuously escalating sophistication in such devices, the demands on the properties and the property control are becoming rather vexatious.
Especially for the electronics industry, improvements of polyimides are needed in forming tough, pin-hole free coatings, having low linear coefficient of thermal expansion, among others. It is not usually possible to maximize all properties, since many of them are antagonistic. Thus, only a compromised solution is usually achieved by at least partially sacrificing one or more of these properties in order to maximize a desired one.
As aforementioned, precursors of polyimides are usually polyamic acids. A typical way of creating a polyimide film or coating is to apply first the polyamic acid precursor, and then heat it to a high enough temperature, usually between 300.degree. C. and 400.degree. C., so that it converts to the polyimide form. This may be undesirable many times for a plurality of reasons. One reason is that the polyamic acid precursor may be such that it does not allow formation of high molecular weight compositions, i.e., when the reactivity of one or more of the monomers (e.g., the diamine) is low. Thus, when the polyamic acid precursor is applied, the coating is very brittle, and it develops unacceptable cracks and other defects. A second reason is that the reaction to form the poly(amic acid) is an equilibrium reaction. That is, although the reactivity of the monomers may be good and a high molecular weight poly(amic acid) can be formed, a small amount of depolymerization (the reverse reaction) can occur which becomes more favored with increasing temperature. Thus, during thermal curing of the poly(amic acid) to polyimide some depolymerization may occur which results in a decrease in molecular weight. If this decrease is significant enough, the film may become brittle to the point of cracking prematurely before total conversion is reached, thus resulting in poor film quality. Another problem associated with this reverse reaction is the susceptibility of the poly(amic amic) chemistry to hydrolysis, so that water present in the poly(amic acid) solution can lead to molecular weight degradation with time and the problems associated with it , e.g., brittleness, cracking. The imide linkage, on the other hand, once formed is generally not susceptible to such reverse reaction. Poly(amic acid)s can also slowly imidize under mild conditions such that shelf life may be limited and refrigeration required. A third reason is that for some applications the high curing temperatures often necessary to convert the poly(amic acid) to polyimide may be detrimental to the substrate or component that the polyimide is used with. A fourth reason is that the liberation of water during the conversion of the polyamic acid to the polyimide may be undesirable, especially in the case of thick coatings. Water liberation in some occasions may cause bubbling and blistering of the coating with catastrophic results to the device comprising the coating.
Therefore, a need has been created for polyimides which are soluble in commonly used solvents (for polyimides), and which do not have to pass through the polyamic acid phase in the stage of producing a coating or other structures, such as fibers, for example, Often, the mechanical properties of such a coating made directly from a polyimide are improved over those of the same composition prepared via a poly(amic acid) precusor. Of course, in addition to being soluble, these polyimides should impart to the coating at least one additional property required by the application. Examples of such desired properties are high modulus, low linear coefficient of thermal expansion, low dielectric constant, low moisture absorption and the like. Typically, it is preferable that the polyimide be soluble at room temperature for ease of handling and processing. However, high temperature processing may not pose a significant problem in some applications, e.g. spinning of fibers.
An especially important property for electronics, and other applications as well, is low thermal expansion coefficient. This is because in electronic components, differences in the expansion coefficients of the components that make up the electronic device can generate stresses in the device which may lead to premature device failure. As electronic components become ever smaller, control of stress becomes an ever greater concern, such that the thermal expansions of the various components of a device should be matched as closely as possible. Since the stress can generally be related to the product of the difference in thermal expansion of the components and the moduli of those components, control of these factors is important in minimizing stress. Polymers generally have much higher thermal expansion coefficents than other components which make up electronics devices, e.g. silicon, silicon dioxide, copper, aluminum, etc., so that often the large mismatch between polymer and the other components of the device can lead to high stresses within the device. Attempts to reduce the stresses between polymers and the other materials have generally focused on reducing the thermal expansion coefficient mismatch between materials, although it is also possible to reduce stress by reducing modulus. In polyimides, low thermal expansion coefficient has generally been achieved by the use of a very stiff, rod like backbone. A good example of this is the polyimide based on 3,3'3,4-biphenyltetracarboxylic dianhydride (BPDA) and p-phenylene diamine (PPD). This polyimide, depending on processing conditions, can have a linear thermal expansion coefficient in the range of 3-4 ppm which closely approximates that of silicon such that the stress between silicon and polyimide can be very low. The drawback of BPDA/PPD and similar polyimides is that they are usually very insoluble in typical solvents such that they generally must be processed from the poly(amic acid) precusor. On the other hand, soluble polyimides are known in the art, but generally to achieve solubility, flexibility is typically incorporated into the backbone structure such that an undesirably high linear thermal expansion is typically obtained and the glass transition temperature is often reduced. It is very desirable therefore to prepare soluble, rod-like polyimides which achieve the advantages of solubility while providing low thermal expansion coefficient and high glass transition temperature. The present invention seeks to provide such a combination. It should be noted here that, to some extent, the linear thermal expansion coefficient that is obtained for a polyimide film depends on processing conditions, such as solution concentration, spin coating speed, viscosity, cure profile and film thickness. This can cause the CTE of a particular polyimide structure to vary over a range; but in general, the rigidity of the polymer chain will determine in what range the CTE will fall.
Polyimides based on phenylated pyromellitic dianhydride and/or phenylated diamines, which polyimides have a rod-like structure, have been prepared by Harris and Hsu, "Synthesis and Characterization of Polyimides Based on 3,6-Diphenylpyromellitic Dianhydride", High Performance Polymers, Vol. 1, No. 1, pp. 3-16, 1989; and by Harris and Sakaguchi, "Soluble Aromatic Polyimides Derived From New Phenylated Diamines", ACS Polymeric Materials Science and Engineering, Vol. 60, pp. 187-191, 1989. Their technique is based on using straight (unbent) dianhydride/diamine structures having bulky pendant groups.
It has now been found that polyimides based on 9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride and a certain class of diamines provide pseudo rod-like polyimides which are soluble and suitable for compositions which may be used to form dielectric films and coatings for electronic circuits characterized by high thermal stability, high glass transition temperature, high modulus, low linear coefficient of thermal expansion, and relatively low dielectric constant. Actually, these polyimides are also suitable for completely different applications, such as for example highly oriented fibers, which are characterized by high modulus and strength.
Since 9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride has a slight bend in its molecule, about 10 degrees, due to the difference in C--C and C--O bond lengths of the central ring unit, it tends to help the solubility without greatly deterring from the rod-like structure. Additionally, the rings do not lie in a plane, but are puckered slightly out of plane by about 22.degree. with the phenyl group of the bridge unit located toward the inside of the pucker. For these reasons, the term "pseudo rod-like" was adopted. Further, in addition to the asymmetry caused by the different bridging groups C--O--C and C--C(CF3)(Ph)--C, further asymmetry is caused along the polymer chain by the random orientation of the phenyl and CF3 groups along the backbone. Generally, they are believed to be arranged atactically (randomly) on either side of the polymer chain. This combination of unusual attributes allows optimized solubility/rod-like structure balance. Even groups which decrease solubility may be introduced up to a certain limit without insolubilizing the polyimide. Other groups, like for example perfluorinated groups, are important for lowering the dielectric constant and decreasing moisture absorption, and can be introduced up to a certain limit without detracting from the desirable properties. In addition, the amount of pendant bulky groups needed to solubilize the polyimide is reduced. It is worth noting that conventional monomers, if they are bent, the angle of the bend is rather high, and therefore they do not share the advantages that 9-aryl-9(perfluoroalkyl)-xanthene-2,3,6,7-dianhydride provides. In addition, the presence of the perfluoroalkyl group of 9-aryl-9(perfluoroalkyl)xanthene-2,3,6,7-dianhydride provides further advantages in that it serves to reduce dielectric constant and potentially moisture absorption in the polyimide as is typically encountered in the art for fluorinated polymers.